CBSE Class 12 Physics 2020 Delhi Set 3 Paper

Question : 3 of 7

Marks: +1, -0

SECTION - C

(a) Define the term 'half-life' of a radioactive substance.
(b) The half life of ‌92238U undergoing alpha decay is 4.5×109 years calculate the activity of 5g sample of ‌92238U.

Solution:

(a) Half-life : The half-life of a radioactive substance is the time required for half the number of the nuclei to decay.
(b) Half-life =4.5×109 years
‌ Mass number ‌‌=238
λ‌=‌

0.693

=‌

0.693

4.5×109

‌ per year ‌
N‌=‌

5×6.02×1023

238

Hence, the activity =A=λN
‌‌ Or, ‌‌‌A=‌

0.693

4.5×109

×‌

5×6.02×1023

238

‌∴A=0.00195×1015 =1.95×1012‌ per year ‌

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